A sync target is the link between incident.io and a single Slack user group,
used to keep that group's membership in step with who is currently on call.
A target identifies the group by its Slack user group ID and Slack team ID,
and remembers whether the incident.io bot should be added to the group so it
can manage membership. On its own a target does nothing: you link it to a
schedule by creating a schedule sync rule (see the Schedules service), and
that rule decides which schedule members flow into the group. As the
schedule's shifts change hands, we update the Slack user group to match.
A single target can be referenced by sync rules on several schedules at once;
linked_schedules lists every schedule with an active rule pointing at it.
Whether the incident.io bot should be added to the group as a member. This is needed for some Slack configurations to let us manage the group's membership.